Ocean warming is the long-term increase in ocean temperatures driven by the absorption of excess heat from greenhouse gases. The ocean acts as Earth’s primary heat sink, storing an estimated 91% of all the extra heat energy trapped in the climate system by human-caused emissions. That means the vast majority of global warming isn’t happening in the air around us. It’s happening underwater.
Why the Ocean Absorbs So Much Heat
Water has an enormous capacity to store thermal energy compared to air. The same amount of heat that would dramatically raise atmospheric temperatures causes a much smaller temperature change when spread through the ocean’s massive volume. This has effectively buffered the planet from the full impact of greenhouse gas emissions, but it comes at a cost: the ocean itself is steadily warming from the surface down.
Most of this heat accumulates in the upper 700 meters, where sunlight penetrates and wind-driven currents mix warm surface water downward. But warming doesn’t stop there. Measurements now track heat changes down to 2,000 meters and beyond, and projections suggest heat will continue penetrating deeper layers through at least 2050. The deep ocean, below 2,000 meters, is warming more slowly but stores an increasingly significant share of the total energy.
How Scientists Track It
The primary tool for monitoring ocean temperatures is the Argo float network, an international array of over 15,000 free-drifting devices deployed across the world’s oceans. Each float sinks to a parking depth of about 1,000 meters, drifts with the currents, then descends to 2,000 meters every 10 days before rising to the surface. During that ascent, it records a vertical profile of temperature and salinity, transmitting the data via satellite.
A newer generation of Deep Argo floats can reach 6,000 meters, covering the full ocean depth. The program’s expanded design aims to maintain roughly 4,000 standard floats, 1,200 deep floats, and 1,000 floats equipped with sensors for biological and chemical measurements. Before Argo launched in the early 2000s, ocean temperature data came mostly from ships and sparse buoy networks, leaving enormous gaps. The float network transformed scientists’ ability to see warming trends in real time across the globe.
Recent Temperature Records
Global sea surface temperatures have been breaking records at an accelerating pace. August 2025 ranked as the third warmest August on record globally, behind only August 2024 and August 2023. In the Arctic’s Atlantic-facing seas, the trend has been especially striking: parts of the Barents and Kara Seas recorded August temperatures as high as 12°C, with some areas running up to 7°C above the 1991–2020 average. The Kara Sea hit its warmest August ever recorded.
These aren’t isolated spikes. They reflect a pattern of sustained warming that has been building for decades, with the most dramatic acceleration occurring in recent years.
Sea Level Rise From Thermal Expansion
When water warms, it physically expands. This thermal expansion is one of the major drivers of rising sea levels, separate from the melting of glaciers and ice sheets. Since 2004, thermal expansion has accounted for roughly one-third of the global sea level rise measured by satellites. The remaining two-thirds comes primarily from ice loss in Greenland, Antarctica, and mountain glaciers.
This means that even if every glacier on Earth stopped melting tomorrow, sea levels would continue rising simply because the ocean is getting warmer. The expansion effect is slow but relentless, and it will persist for centuries because the deep ocean takes a very long time to equilibrate with surface conditions.
Stronger Hurricanes
Warmer ocean water is fuel for tropical cyclones. Hurricanes draw their energy from the heat stored in the upper ocean, and higher sea surface temperatures mean more energy is available to intensify storms. One important factor is what happens beneath the surface: when a hurricane’s winds churn the water below, they normally pull up cooler water that limits the storm’s strength. But in regions where a warm layer sits atop a distinct barrier that resists mixing, this cooling effect is reduced by about 33 to 36%. The result is that storms intensify roughly 50% faster over these areas compared to regions without such barriers.
As the ocean continues to warm, this dynamic becomes more common and more pronounced. Rapid intensification, where a hurricane’s wind speed jumps dramatically in a short period, has become a growing concern for coastal communities because it leaves less time for evacuation and preparation.
Coral Bleaching and Marine Ecosystem Stress
Coral reefs are among the most temperature-sensitive ecosystems on Earth. Bleaching begins when water temperatures exceed the local summertime average by just 1°C. Scientists track this stress using a metric called degree heating weeks, which combines how far above the threshold temperatures are with how long they stay elevated over a 12-week window.
When accumulated heat stress reaches 4 degree-weeks, significant bleaching occurs, particularly among more sensitive coral species. At 8 degree-weeks or higher, widespread severe bleaching and substantial coral death become likely. With ocean temperatures consistently setting new records, many reefs now face bleaching-level heat stress annually rather than once a decade, leaving too little time for recovery between events.
Oxygen Loss in Warmer Water
Warmer water holds less dissolved oxygen. This is a basic physical property: as temperatures rise, gases become less soluble and escape from the water more readily. But warming also strengthens the layering of the ocean, with lighter warm water sitting on top of denser cold water. This stratification reduces the mixing that normally carries oxygen-rich surface water down to deeper layers.
The combination of reduced oxygen solubility and weaker ventilation is creating expanding zones of low-oxygen water throughout the ocean. Marine heatwaves make this worse, causing sharp temporary drops in oxygen levels that can stress or kill fish, shellfish, and other organisms. For species that need high oxygen levels, like tuna and many reef fish, this represents a shrinking habitat even if temperatures in their range haven’t yet become directly lethal.
Effects on Ocean Circulation
The Atlantic Meridional Overturning Circulation, often called AMOC, is a vast conveyor belt that carries warm surface water northward from the tropics and returns cold, dense water southward at depth. This system helps regulate climate across the Northern Hemisphere, keeping Western Europe significantly milder than it would otherwise be.
Data shows AMOC remained stable from 1955 to 1994 but has weakened over the last two decades. Scientists attribute this slowdown to continued warming of the ocean surface and changes in salinity in the upper layers, both of which reduce the density contrast that drives the circulation. A substantially weaker AMOC would mean less warm water reaching northern regions, potentially making tropical areas hotter while polar regions grow colder. Despite the slowdown, the system shows some resilience, largely because the Gulf Stream that feeds it remains robust.
Projections Through 2100
How much warmer the ocean gets depends heavily on emissions trajectories. Under a low-emission scenario where the world aggressively cuts carbon, global average sea surface temperatures are projected to rise by about 0.9°C (compared to 1995–2014 levels) by the end of this century. Under a high-emission scenario with continued fossil fuel expansion, that increase climbs to roughly 2.9°C, with an upper range as high as 4.1°C.
Middle-of-the-road scenarios project increases of 1.5 to 2.2°C. Every fraction of a degree matters, because ocean warming compounds its own effects: more thermal expansion, more oxygen loss, more energy for storms, and more stress on marine life. The difference between the low and high scenarios represents vastly different futures for coastal communities, fisheries, and ocean ecosystems worldwide.